Heretofore, it has been necessary to open a furnace door, most often for this type furnace there being no hinge but requiring a removal of the entire door from its inserted heat-sealed position and state, or alternately with the see-through door a major degree of heat loss has been a disadvantage, through the transparent heat barrier such as is disclosed in U.S. Pat. No. 3,800,716 for example, where the barrier is located at the end of the see-through channel adjacent the interior of the furnace heating space, such that the transparent barrier becomes highly heated and the air within the channel space exterior to the interior of the furnace space, is in contact with the exterior surface of the transparent barrier and becomes highly heated with resulting air currents such that the heated air escapes to exterior space through the open end of the see-through channel. Also, it has been found in the development of the present invention, that there is a large heat loss in the absence of a proper seal between an insert and a receiving structure defining space communicating with an inner furnace of a muffle furnace such as a porcelain furnace. It has heretofore been a difficult choice between having a provision for viewing within the furnace and retaining heat for quick heating and more efficient operation devoid of viewing capability without having to remove the door.